You only have one pot of willpower. If you use up all your willpower doing professional tasks, exercising, etc then you'll have less willpower to resist over eating. If you use up all your willpower for weight loss, then you'll have less willpower for other tasks.

This article (and other research done by the authors) talks about that sort of thing. This is why diets that work are ones that reduce the amount of willpower required, or shift the willpower requirements to situations where it is easier to control your decision making. For example, a low carb diet works because you simply don't buy carbs at the store, so when you're at home, you don't have to expend willpower when you're hungry and deciding what to eat. Intermittent fasting works because you simply don't eat breakfast.

I eat for pleasure and I have a hard time moderating my intake, so I've worked around this by reducing my breakfast to coffee, yohimbine (caffeine/yohimbine is great for energy and appetite reduction), and fish oil (~2g). Lunch is two cups of milk and two scoops of protein powder as these are all that I have at work, so it is the only option -- no willpower used. This puts my daily calories low enough that I can eat as much as I want for dinner and still lose weight consistently.